test(V2): migrazione test common/

Copiati e aggiornati i test da services/common/tests/ a tests/unit/common/.
Import aggiornati da mcp_common a cerbero_mcp.common. Eliminati test di
funzionalità V1-only (app_factory, environment, auth/Principal, server_base).
Refactored test_audit.py (principal→actor str) e test_mcp_bridge.py
(TokenStore→valid_tokens set). 71/71 test passano.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

tests/unit/common/test_stats.py

@@ -0,0 +1,51 @@
+from __future__ import annotations
+
+import random
+
+from cerbero_mcp.common.stats import cointegration_test
+
+
+def test_cointegrated_synthetic_pair():
+    """Costruisco coppia cointegrata: B random walk, A = 2*B + noise stazionario."""
+    r = random.Random(1)
+    b = [100.0]
+    for _ in range(300):
+        b.append(b[-1] + r.gauss(0, 1))
+    a = [2 * b[i] + r.gauss(0, 0.5) for i in range(len(b))]
+    out = cointegration_test(a, b)
+    assert out["cointegrated"] is True
+    assert out["beta"] == pytest_approx(2.0, rel=0.05)
+    assert out["adf_t_stat"] is not None
+    assert out["adf_t_stat"] < -2.86
+
+
+def test_not_cointegrated_independent_walks():
+    """Due random walk indipendenti → spread non stazionario → no cointegration."""
+    r = random.Random(2)
+    a = [100.0]
+    b = [100.0]
+    for _ in range(300):
+        a.append(a[-1] + r.gauss(0, 1))
+        b.append(b[-1] + r.gauss(0, 1))
+    out = cointegration_test(a, b)
+    # Per due RW indipendenti, t-stat ADF è solitamente > -2.86 → non cointegrate
+    assert out["cointegrated"] is False or out["adf_t_stat"] > -3.0
+
+
+def test_cointegration_short_series():
+    out = cointegration_test([1.0, 2.0], [3.0, 4.0])
+    assert out["cointegrated"] is None
+    assert out["beta"] is None
+
+
+def test_cointegration_mismatched_length():
+    out = cointegration_test([1.0, 2.0, 3.0], [1.0, 2.0])
+    assert out["cointegrated"] is None
+
+
+def pytest_approx(value, rel):
+    """Tiny helper to avoid importing pytest just for approx."""
+    class _Approx:
+        def __eq__(self, other):
+            return abs(other - value) <= abs(value) * rel
+    return _Approx()